The purpose of this study is to examine the role of the host immune response on both acute and chronic viral infections of the central nervous system. These studies will examine the host immune response with relationship to mechanisms of protection as well as disease production in a viral infected host. In addition, the effect of virus on the host immune response will be examined. Specifically, this will include the effect of virus on the functional capacity of both T and B lymphocytes. BIBLIOGRAPHICAL REFERENCES: Burks, J.S., Narayan, O., McFarland, H.F. and Johnson, R.T.: Acute encephalopathy caused by defective virus: I. Studies of newcastle disease virus infections in newborn and adult mice. Neurology 26: 584-588, 1976.